Device and method for projection device based soft proofing

ABSTRACT

A display ( 36 ) for reproducing an image intended for printing on a substrate using a set of inks, the image having a perceived color gamut when print it on the substrate, the display ( 36 ) including a light source ( 38 ) generating a set of at least three primary color (RGB), and a controller ( 42 ) combining the set of at least three primary color to substantially reproduce the image, wherein the at least three primary color define a viewed color gamut which substantially covers the perceived color gamut.

FIELD OF THE INVENTION

An embodiment of the present invention relates to a device, system and amethod of electronic display for digital soft proofing of printedmatter.

BACKGROUND OF THE INVENTION

Reproduction of color involves the creation of an accurate apparentcolor match between original and reproduction. Color originals may be,for example, pictorial slides, which are analog in nature and have avery wide color gamut, wider than in typical reproduction, such asoffset printing. In the age of digital information most of thereproduction process is performed digitally. The original slide isscanned to obtain a file containing the color data in terms of RGBvalues. The file is then converted to CMYK separations, and afterwardsplates are created and installed on a press for print. To obtain colorconsistency, proofs are performed and examined in various stages of theprocess to ensure that each step is color-consistent with its previousstep.

Accurate reproduction of color is very important for printed matter.Typically, in order to achieve an accurate color match, a “hard proof”may be printed on paper and sent to the customer and/or designer forapproval. Upon approval, the proof is delivered to the printing shop,where the printer working on the press machine adjusts the press machineuntil the printed sheets match the hard proof.

For certain applications, most notably in the packaging industry, thereis a need for special colors, many of which are out of the gamut of CMYKprocess inks. Therefore, standard CMYK hard proofers are unsuitable forsuch proofing.

The manual procedure of proofing limits the advantages of digitalworkflow. The need for an accurate digital “soft proof” on an electronicdisplay is clear. Currently available “soft proofing” devices areintended to enable designers and pre-press personnel to view works on adisplay of a computational device such as a personal computer orworkstation. Such devices may be based on Cathode Ray Tubes (CRT) orLiquid Crystal Devices (LCD). The final product, however, is an imageprinted on paper. Currently, soft proofing devices do not overcomeinherent deficiencies of digital print proofing, and in particular donot provide good color match, in the sense that they cannot accuratelyreplicate the colors electronically as they would appear on the printedmaterial. This is a serious drawback, as many printed works are nowtransferred digitally from design to printed material over a network,and any intermediate procedure that must involve printing onto aphysical substrate, prior to the final printing step, significantlyreduces the efficiency of the printing process.

It should be noted that, for various reasons, CRT color displaysgenerally do not provide an accurate color match to a printed image. Asshown in FIG. 1, the color gamut reproduced by a printing press, e.g.,an offset printer, is different from that of CRT displays, e.g., thereare non-overlapping regions in the printed gamut relative to the CRTgamut and vice versa. Thus, the colors that can be displayed by a CRTmonitor do not overlap the colors that can be produced by printingmethods. For example, FIG. 1 illustrates the color gamut of offsetprinting under D50 daylight illumination as compared with the colorgamut that can be displayed on a typical CRT monitor. The CRT monitorcannot reproduce the yellow colors, e.g., in a vicinity of colorcoordinates (x-0.45,y=0.45), nor can the CRT monitor reproduce a widecyan-green spectrum, e.g., in a region between color coordinates(x=0.2,y=0.2) and (x-0.3,y=0.5). It is evident that a CRT monitor cannotreproduce certain printing colors that have color coordinates outsidethe CRT gamut. At least part of the limitation of the CRT gamut isassociated with the physical properties of the CRT screen, e.g., theemitted spectrum from the red, green and blue phosphors of the CRT.Furthermore, in many cases printers use special colors (e.g., Coca-ColaRed or other trade mark colors like Pantone, Toyo BS, etc.), many ofwhich are outside the gamut of the CRT and the offset CMYK process inks.These colors cannot be matched by CRT monitors, nor can they be matchedby a “hard” proofer designed to match process CMYK inks.

A further problem with soft proofing on CRT monitors is that the colorsof the inks (and color combinations of such inks) are different fromthose reproduced by the RGB phosphors of CRT monitors, and therefore aspecial transformation from CMYK values to corresponding ROB values maybe required in order to reach colors closer to an apparent calorimetricmatch. Such transformation is the basis of existing methods of colormatching in general, and soft proofing on display methods in particular.Such methods are based on mapping of the color space of an output device(e.g., printing press, display) onto a device-independent color space,such as L*a*b*, as defined by Commission Iternationale De L'éclairage(CIE). Using this mapping, a multi-dimensional transformation from theRGB space of the display into the L*a*b* space may be performed. Then,another transformation from the L*a*b* space onto the CMYK space of theprinting press may be performed. These transformations, known asprofiles, are performed on the data file containing the work, beforeprinting, by a color management system. The International ColorConsortium (ICC) has standardized this method for color matching.

It is noted that the profiling process described above maps the colorscreated by CMYK inks printed on a certain substrate and viewed undercertain light conditions onto a color space of the RGB phosphors. Thespectra of the light reflected off the CMYK inks depends on the lightingconditions, e.g., the spectrum of white light which illuminates thepaper of the printed material, and on the reflectivity of the paper.Therefore, different profiles may be required for each combination ofpaper type and/or ink type and/or illumination conditions, resulting ina cumbersome profiling process.

Furthermore, CRT monitors may not have adequate color consistency andstability, for a number of reasons. First, the electronic circuitry thatdrives the electron beam is not sufficiently stable, resulting inchanges in the brightness of the light emitted from the phosphors.Furthermore, the ratio between the brightness of the Red, Green and Blue(RGB) light may also change, resulting in color variations for a givenRGB input value. CRT displays are also highly influenced by externalconditions such as magnetic fields. The presence of even slightlymagnetized materials near a CRT monitor (such as a loudspeaker, motor,etc) will cause color shifts that are beyond the acceptable level inproofing applications. Thus, a CRT for proofing applications should beused in a highly controllable environment. This dictates the use ofspecially calibrated and electronically stabilized monitors, such as theReference and Personal Calibrator™, available from Barco, Kortrijk,Belgium. Furthermore, the exchangeability of CRT monitors is verylimited, because the phosphors have tolerances in their emissionspectra, resulting in different colors for different CRT units. Inaddition the phosphors decay and fade over time. All these phenomenarequire continuing calibration and profiling of the CRT itself.

Moreover, even if the CRTs could be made more stable, they are typicallyunsuitable for color reproduction. Color accuracy is highly dependent onambient light conditions. Small amounts of ambient light, reflected fromthe CRT screen, are added to the light originating from the phosphors,altering the overall appearance of the displayed picture. This effect isvery pronounced when viewing relatively dark colors, but even brightercolors are vulnerable to this effect. Since the brightness of the CRTused as computer monitors is typically relatively low, the level ofambient light in a normal working environment is sufficiently high tocause unacceptable color shifts. Thus, the use of CRT for proofingpurposes dictates the use of a controlled environment, e.g. a relativelydark room. Further, the high reflectance of “shadow mask” technologiesused by many CRT monitors exasperates the color variation problems ofexisting displays.

The match between images depends also on their level of brightness. Asdiscussed above, the brightness of normal CRT used as computer monitorsis relatively low. Enhancement of the brightness of CRT based computermonitors is limited, because the emission of the light from thephosphors is associated with a harmful X-ray radiation created by thedeceleration of electrons impinging on the screen. In proofingapplications, however, hard prints are typically viewed under a muchhigher light intensity to maximize image brightness. To compensate forthis difference in lighting conditions requirement, attempts have beenmade to account for different levels of illumination of the print andthe CRT, by including perceptual models in the mathematicaltransformation of the CMYK data to the RGB input. Unfortunately, no suchtransformation has produced satisfactory results, partly because thetransformation correction depends on human perception, which does nothave an established mathematical model. Therefore, it is practicallyimpossible to compare a printed sample to a soft proof viewed on a CRTunder the same ambient illumination level.

As described above, the RGB spectra reproduced by CRT phosphors is verydifferent from that of color inks and their overlaps. Moreover, inviewing the subtractive color combinations produced by color inks, thenumber of elementary colors integrated by the eye is larger than that ofthe standard RGB system. Certain colorimetric match to “in-gamut”colors, as described above, may be possible; however, even if goodcolorimetric match between print and monitor may be achieved for oneobserver, such a match is not guaranteed for another observer. This isdue to the fact that color is a psychophysical phenomenon, whichinvolves the spectral input to the eye, the optics of the eye, and aperceptual process. Different individuals are likely to differ in theircolor perception, due to variations in the eye physiology. Thesignificant spectral discrepancy between the CRT phosphors and theprinted ink elementary colors often results in situations where a matchbetween CRT and print may be reasonable for one observer, but notacceptable for another observer. Due to this and other deficiencies, aCRT monitor cannot be used as an accurate device for colorcommunication.

Many attempts have been made in the past to adapt CRT displays for softproofing using calibrated monitors and ICC profiling, e.g., AppleColorSync™ from Apple Computer Inc., CA, USA, Barco Calibrators fromBarco, Kortrijk, Belgium, Virtual Matchprint™ from Kodak PolychromeGraphics (KPG), etc. However, CRT based soft proofing has not gainedsufficient ground in the industry, mainly due to the deficienciesdiscussed above.

Other display technologies, e.g., LCD displays of laptop and desktopcomputers, suffer from some of the problems discussed above, in varyingdegrees. Furthermore, the color gamut of most flat-panel LCD displays issmaller than that of CRT and, therefore, such displays cover even asmaller fraction of the printed color compared to CRT displays.Additionally, LCD displays have a high variation of color and brightnessas a function of viewing angle, whereby a slight change in the viewingangle of the observer may result in significant changes in color.

SUMMARY OF THE INVENTION

Embodiments of the present invention provide devices, systems andmethods of electronic color display that enable accurate soft proofingof images under normal ambient lighting conditions.

Embodiments of one aspect of the present invention provide a display forreproducing an image intended for printing on a substrate using a set ofinks, the image having a perceived color gamut when printed on thesubstrate, the display including a light source generating a set of atleast three primary colors, and a controller combining the set of atleast three primary colors to substantially reproduce the image, whereinthe at least three primary colors define a viewed color gamut whichsubstantially covers the perceived color gamut.

Embodiments of another aspect of the present invention provide a methodfor reproducing an image intended for printing on a substrate using aset of inks, the image having a perceived color gamut when printed onthe substrate, the method including accepting data corresponding to theimage, converting the data to data corresponding to a set of at leastthree primary colors, selectively producing light of the at least threeprimary colors, and combining the at least three primary colors tosubstantially reproduce the image, wherein the at least three primarycolors define a viewed color gamut which substantially covers theperceived color gamut.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter regarded as the invention is particularly pointed outand distinctly claimed in the concluding portion of the specification.The invention, however, both as to organization and method of operation,together with objects, features and advantages thereof, may best beunderstood by reference to the following detailed description read inconjunction with the accompanying drawings, in which:

FIG. 1 is a schematic illustration of a comparison between a typicaloffset printing color gamut and a typical color gamut of a prior art CRTdisplay;

FIGS. 2A and 2B are schematic block diagrams of embodiments of anexemplary display device and system according to an embodiment of thepresent invention;

FIG. 3A is a schematic illustration of transmission spectra for a set offilters that may be used in accordance with some embodiments of thepresent invention;

FIG. 3B is a schematic of a comparison of a set of transmission spectraand their corresponding reproduction according to exemplary embodimentsof the present invention;

FIG. 4A is a schematic illustration of the spectra of a lamp currentlymarketed as the Osram™ VIP lamp;

FIG. 4B a schematic illustration the spectra of a D50 equivalentfluorescent lamp;

FIG. 4C is a schematic illustration of a spectrum of a correction filteraccording to an embodiment of the present invention;

FIG. 4D is schematic illustration of a spectrum created using acorrection filter according to an embodiment of the present invention.

FIG. 5 is schematic illustration of a comparison, on a chromaticitydiagram, between the gamut obtained by an electronic color display usingfilters selected in accordance with exemplary embodiments of the presentinvention and a typical color gamut of color printing;

FIGS. 6A and 6B are schematic illustrations of exemplary designs oftransmission spectra for a spectrum correcting filter according toembodiments of the invention; and

FIG. 7 is a schematic illustration of transmission spectra for a set offilters that may be used in accordance with further embodiments of thepresent invention.

It will be appreciated that for simplicity and clarity of illustration,elements shown in the figures have not necessarily been drawn to scale.For example, the dimensions of some of the elements may be exaggeratedrelative to other elements for clarity. Further, where consideredappropriate, reference numerals may be repeated among the figures toindicate corresponding or analogous elements.

DETAILED DESCRIPTION OF EMBODIMENTS THE INVENTION

Various aspects of the present invention are described herein. Forpurposes of explanation, specific configurations and details are setforth in order to provide a thorough understanding of the presentinvention. However, it will also be apparent to one skilled in the artthat the present invention may be practiced without the specific detailspresented herein. Furthermore, certain well-known details or featuresmay have been omitted or simplified in order not to obscure the presentinvention.

A monitor for print proofing in accordance with embodiments of theinvention may be constructed, for example, as described in InternationalPatent Application PCT/IL01/01179, entitled “SPECTRALLY MATCHED PRINTPROOFER”; filed Dec. 18, 2001, and published Jun. 27, 2002, as WO02/50763, the entire disclosure of which is incorporated herein byreference. While methods and systems disclosed in this patentapplication are used in embodiments of the present invention, forexample, methods to convert source data to primary data, or methods ofcreating primary colors, e.g., filters, it will be appreciated that, inalternate embodiments, the system and method of the present inventionmay be used with other types of display technology.

According to one embodiment, the invention includes a projection displayusing three primary colors. It will be apparent to those skilled in theart that a triangular color gamut produced by the three-primary deviceof the invention contains the gamut of typical CMYK process inks. Asdescribed in detail below, the display may include a low-reflectionviewing screen. The display of the invention may further include a setof filters that may be used to reproduce various illuminationconditions. Further, the display may include a set of filters that maybe used to reproduce various reflectance properties of the printsubstrate. Additionally or alternatively, the display may include colormanagement tools and software to obtain a desired color match relativeto a printed substrate. The display of the invention may include a colorcalibration system and may be combined or used in conjunction with alight box.

Embodiments of the present invention include a system and method forsoft proofing using a non-CRT projection type monitor. The projectiontype monitor may be based on a polychromatic light source, such as, forexample, a high pressure Mercury lamp, Xenon lamp, or any other suitabletype of light source, which illuminates a spatial light modulator (SLM),for example, a DLP type SLM. Color may be achieved via filters areplaced in the optical path of the polychromatic light to createdifferent primary color patterns. The different color patterns may beseparated spatially or temporally, and the full color sensation isobtained by spatial or temporal integration of the separate patterns bythe eye of a viewer. Other types of lamps or modulators may be used inalternate embodiments of the invention.

Referring now to the drawings, FIGS. 2A and 2B are schematic blockdiagrams of embodiments of an exemplary display device and system forelectronic soft proofing according to an embodiment of the presentinvention. FIG. 2A shows a basic embodiment of the exemplary displaydevice and system, while FIG. 2B shows an embodiment featuring a lightprojection mechanism.

As shown in FIG. 2A, a system 36 according to one embodiment features alight source 38 for producing light of at least three primary colors. Inone embodiment, these colors include specific selections of RGB primarycolors, as described in detail below, which results in a color gamutthat contains substantially the entire gamut of process inks printed ona substrate. The gamut of process inks is defined by its elementarycolors, i.e., the cyan magenta and yellow inks, and the overlap betweenthe inks, namely, red (overlap of magenta and yellow), green (overlap ofcyan and yellow) and blue (overlap of cyan and magenta), as viewed onthe printed substrate under certain illumination conditions. Theelementary colors, e.g., cyan, magenta, yellow, red, green and blue,determine the boundaries of the perceived gamut obtained by a printingprocess. The primaries of the display in accordance with embodiments ofthe invention are chosen to cover the printed gamut or even, in someembodiments, a wider gamut. In one embodiment, one filter or primarysource may be used for each elementary color; in alternate embodimentslower numbers of primaries may be mixed in the proper proportions toreproduce with acceptable accuracy a higher number of elementary colors.The light from light source 38 is displayed on a viewing screen 40,thereby enabling the human viewer to see the colors of the displayedimage (not shown). Preferably, the light from light source 38 isprojected onto viewing screen 40. In order for each color to be properlydisplayed in the correct location of the displayed image, a controller42 controls the production of light of each color, such that the correctlight is shown at the correct location of viewing screen 40. Controller42 may be separate from light source 38, such that these two componentsare not combined into a single component. In alternate embodiments ofthe invention, the two components may be provided in the same device.

Although not shown or described herein, in alternate embodiments of thesystem and method of the present invention, the specific primary colorsdefined below may be produced by other methods, such as by LCDs or LEDs.

Various physical print systems may be used to convert print data such asdigital CMYK print data to printed material. For example, data may beconverted by printers using silkscreen methods, lithography, ink jetmethods, or other methods. Each method may result in a differentappearance for identical input data. The system and method of thepresent invention may simulate various print methods. In one embodiment,the same display may be used to simulate different printing methods. Auser selectable setting may control various aspects of the display, suchas filter settings or software or hardware controls, such as dataconversion or spectral correction or combination methods, to allowdifferent print processes to be simulated.

In one embodiment of system 36, light source 38 projects light of atleast three colors, without being able to control the location of theprojected light onto viewing screen 40. Controller 42 then determinesthe relative location of light of each color as projected onto viewingscreen 40, for example with a spatial light modulator and/or anothersystem of mirrors and/or lenses.

In order for controller 42 to be able to determine the correct light forbeing displayed at each portion of viewing screen 40, controller 42optionally receives data from a data input 45, which may optionally bedigital or analog. Most preferably, controller 42 also receivesinstructions and/or commands from a converter 46, which is functionallyconnected between data input 45 and controller 42. Converter 46 convertsthe data from data input 45 into a format suitable for controller 42,and also includes any necessary instructions and/or commands forenabling controller 42 to be able to understand the data. Converter 46may be implemented in software, hardware, or any suitable combination ofsoftware and/or hardware. Optionally, converter 46 may also convert thedata from an analog signal to digital data, such that controller 42 isonly required to receive digital data. Converter 46 may be able todetermine the appropriate combination of light of at least threedifferent primary colors in order to accurately represent the colorimage data with displayed colors which match or substantially match thecolors of a certain printed material, such that the appearance of thedisplayed image matches or substantially matches the appearance of acertain set of inks as printed onto the paper of the printed material.

In alternate embodiments, converter 46 is able to determine theappropriate combination of light of another number of primary colors inorder to accurately represent a set of elementary colors (e.g., cyan,magenta, yellow, red, green, blue) and the “white” color of thesubstrate. For example, three or four primaries may be combined toreproduce these seven colors. In other embodiments, a different numberof elementary colors may be reproduced, for example, if proofing isdesired for an ink system that produces a different number of elementarycolors. The match may be based on spectral resemblance, in the sensethat the primary filters may be chosen so that the reflection spectra ofthe elementary color inks, their overlaps, and the substrate, may bereproduced. For example, three or four primaries may be combined toreproduce the seven spectra of the inks, their overlaps and thesubstrate.

FIG. 2B shows an embodiment of an exemplary display device according toan embodiment of the present invention, which is based on a sequentiallight projection system, similar in certain respects to that suggestedin U.S. Pat. No. 5,592,188, which is hereby incorporated by reference asif fully set forth herein. Embodiments of the present invention use aset of primaries whose colors span a gamut sufficiently wide to containthe entire gamut of colors produced by light reflected off printinginks, and/or overlaps of inks, and substrates of a given printingprocess. Embodiments of the present invention may use a set of primarieswhose spectra are able to substantially reproduce the transmissionspectra produced by light reflected off printing inks, and/or overlapsof inks, used in a given printing process. One embodiment may useprimary colors of spectra similar to the transmission spectra reflectedoff printing inks, and/or overlaps of inks, to accurately display theimage which is to be printed onto a “hard copy” of printed material,such as, for example, paper. Other embodiments may use combinations ofprimaries to reproduce such transmission spectra.

A system 48 according to one embodiment of the invention is based onpassing white or substantially white light from a source 20 through aspectrum-correcting filter 22 in order to attempt to match the spectrumof the light to at least one of, and more preferably both of, therelevant required illumination conditions and the relevant paper, orother printing substrate, reflectance spectrum, as described in detailbelow. Spectrum-correcting filter 22 may optionally include twofunctional components: a first functional component for correcting thespectrum of light with regard to the required illumination conditions;and a second functional component for correcting the spectrum of lightwith regard to the relevant printing substrate reflectance spectrum.These two functional components are optionally implemented as twoseparate parts of spectrum-correcting filter 22, but alternatively maybe implemented in a single physical device, e.g., in a single set offilters whose transmission spectra account for both types of correction,as described below.

Such correction filters may be implemented in various manners. Forexample, a filter or set of filters including correcting for severaldifferent spectra may be included. A filter wheel or filter bar withsuch filters may be included, and a user may adjust which filter, ifany, may provide correction. In one embodiment, the printing substratecorrection filter is a continuously variable filter and the illuminationcorrection filter includes discrete filters. In other embodiments, nosuch correction is needed, and other types and combinations ofcorrection or adjustment may be used.

The brightness of the light is optionally and preferably controlled byadjusting the amount of power supplied by a power supply 23 or by avariable neutral density filter (not shown) or by a variable sizemechanical iris in the illumination path (not shown). The spectrallycorrected light passes through appropriate color filters 52 to formcolored light of a defined spectral range. As previously described,system 48 may use at least three such colored filters 52, e.g., threeprimary colors, and a “white” neutral density filter, which as shown mayoptionally be configured in a color filter wheel 24, but may optionallyinclude other numbers of filters or primary colors.

In further embodiments, primaries are reproduced using methods otherthan filters; for example, different LEDs may provide primaries.

In order for the light to be directed through the appropriate filter 52,preferably the light is focused by a condenser lens 21, optionallyimplemented as two such lenses 21 for the purposes of illustration onlyand without any intention of being limiting. In alternate embodiments,various components, such as the condenser, may be eliminated. Thefocused light is then directed through one of the filters on filterwheel 24, which holds the color filters 52. In this example, thecombination of light source 20, spectrum-correcting filter 22 and colorfilters 52 can be considered to form at least part of the light sourceof FIG. 2A above, optionally with other components involved in theproduction of the light itself.

Preferably, the colored light illuminates a spatially modulated mask 26,also known as a Spatial Light Modulator (SLM) which determines theparticular color to be displayed at each portion of the image, typicallyaccording to each pixel, by determining whether light of that color ispermitted to pass for illuminating that pixel. For example, a digitalmicro-mirror device (DMD) such as, for example, that available fromTexas Instruments, or a Ferroelectric Liquid Crystal (FLC) SLM such as,for example, that available from Displaytech, or any other suitable SLMdevice as is known in the art, may be used.

The colored light for this image may then be projected by a projectionlens 28 onto a viewing screen 29. Viewing screen 29 displays theresultant colored image to the user (not shown). Spatially modulatedmask 26, and preferably the combination of spatially modulated mask 26and projection lens 28, can be considered to be an example of thecontroller from FIG. 2A. In alternate embodiments other controllers andmethods for controlling the system may be used.

A motor 63 may rotate filter wheel 24 in the path of light emanatingfrom light source 20, so in each turn spatially modulated mask 26 isilluminated sequentially by the colors in filter wheel 24. The rate ofrotation may be at the frame frequency, which is the frequency at whichthe full-color image is refreshed on viewing screen 29.

Preferably, the loading of the data into spatially modulated mask 26 issynchronized by a timing system 207, according to the rotation of filterwheel 24. The light beam is spatially modulated by spatially modulatedmask 26, so that the apparent brightness of each primary color varies atdifferent portions of viewing screen 29, typically according to eachpixel of the image. Each position on viewing screen 29 may be associatedwith a certain pixel of spatially modulated mask 26. The brightness ofthat position is determined by the relevant pixel data in the image. Thevalues for the pixels of the image are optionally retrieved from animage data file 201. In accordance with embodiments of the presentinvention, the image data values are converted by a converter 203 into asignal representing the data in a three-primary-color format, e.g., aRGB format, corresponding to the primary colors as defined and describedherein. The converted three-primary-color data may then be collected bya Frame Buffer and Formatter 206, which may rearrange the data and/oradjust physical parameters of the signal into a format suitable forcontrolling SLM 26 or any other device that may be used to control thereproduced image.

In accordance with embodiments of the invention, the human viewertemporally integrates the sequential stream of the primary images toobtain a color image, which spectrally matches or substantiallyspectrally matches the image on paper. In further embodiments, othermethods of producing primaries and displaying primaries may be used, andother light delivery mechanisms using different sets of components maybe used. For example, an SLM need not necessarily be used.

Exemplary implementations of various components of the above projectiondisplay system are now given in greater detail. Spectrum-correctingfilter 22 may correct for at least one of, and preferably both of, therequired illumination conditions and the relevant paper or printingsubstrate reflectance spectrum. One standard illumination in theprinting industry is D50 illuminant, or D65 in United States,representing daylight illumination from a black body at 5000° K and6500° K, respectively, through the atmosphere. Other illuminants, suchas illuminant A, tungsten lamp, typical for indoor illumination, and9300° K illumination, typical for light outdoors under a blue skywithout direct sunlight, are also common. The lamp itself has aspectrum, which is typically, but which may not be, very different fromthese illuminants and may depends on the type of the lamp, e.g.tungsten, halogen, metal-halide, Xenon and others. Therefore, thespectrum of the emitted light may be corrected.

The illumination correction is preferably obtained by placingspectrum-correcting filter 22 with a transmission spectrum T_(f)(λ)after light source 20. The filter spectrum is given by T_(f)(λ)?S_(i)(λ)/S_(L)(λ), where S_(L)(λ) and S_(i)(λ) are light source 20 andthe required illuminant spectra respectively. The light passing throughspectrum correcting filter 22 has a spectrum preferably identical orsubstantially identical to that of the required illuminant. These typesof filters are based on color temperature conversion filters incombination with narrow notch filters, which are applied if the lampspectrum contains narrow spectral lines, which may be rejected. Thedesign of such filters is known in the art.

FIGS. 4A-4D depict spectra of various white lights, a correction filter,and the spectrum resulting from the use of the correction filter,according to an embodiment of the present invention. FIG. 4A depicts thespectra of a lamp currently marketed as the Osram™ VIP lamp. FIG. 4Bdepicts the spectra of a D50 equivalent fluorescent lamp. A correctionfilter having a spectrum as shown in FIG. 4C may be applied to thespectrum of FIG. 4A to produce the spectrum of the solid line in FIG.4D, which substantially matches the spectrum of FIG. 4B. Both spectrahave substantially the same color temperature and a high color-renderingindex. In alternate embodiments, other correction spectra may beapplied.

FIG. 6A schematically illustrates an exemplary spectrum for correctingfilter 22 (FIG. 2B) according to one embodiment, suitable for correctingfor paper reflection, and construction of a filter for such spectrum. Ayellowish tint may be obtained for example, using a long-wavelengthcutoff filter with density in the range of 0-0.1, as shown schematicallyin FIG. 6B. In embodiments of the invention, the light impinges on arelatively small area of the filter. The blue part of the spectrum maybe enhanced or reduced according to the relative density in the bluepart of the spectrum with respect to the rest of the spectrum at theposition of incidence. The color of the transmitted light may beadjusted by shifting the placement of the filter along the x-direction,thereby changing the relative density of the filter that filters thelight, until the white area on the screen has the same color as that ofthe paper. In further embodiments of the invention, other spectrumcorrecting filters and/or methods may be used. Such filters or methodsmay be operator-selectable or adjustable to enable correction formultiple light sources or papers.

Embodiments of the present invention may use of a modified projectiondisplay. The projection display may be equipped with specially designedcolor filters, as described below, a specialized viewing screen with lowreflection and a wide viewing angle and, optionally, a set ofillumination matching filters.

To obtain an accurate reproduction of colors, the gamut of the displayin accordance with embodiments of the invention is sufficiently wide tocover, or substantially cover, the entire gamut of analog or digitalhard proofing devices, such as, for example, Analog and DigitalMatchprint, available from Kodak Polychrome Graphics (KPG), Analog andDigital Cromalin, available from DuPont, etc., as well as a color gamutaugmented by various special colors that may be used in the printingindustry. Because the colors in the display according to embodiments ofthe invention may be produced by filters, and not by phosphors, thegamut covered by the projection device is not limited to that commonlyused in CRT monitors. Filter technologies, for example, interferencefilters, enable relatively straightforward realization of substantiallyany transmission curve desired. Therefore, complete control over thegamut coverage may be achieved, allowing for instance, spanning of theentire gamut of CMYK inks, e.g., using three, appropriately selected,primary colors. In some embodiments, filters may also be added toconvert the lamp spectrum to a required or desired illuminationspectrum.

As shown schematically in the chromaticity diagram of FIG. 5, in a threeprimaries display according to embodiments of the present invention, thecolor filters may be set in such a way that the triangular area defiedby the display pries substantially encloses the entire hexagonal areadefined by standard CMYK inks. Since there are various transmissionspectra for each filter (and thus for each corresponding primary) thatmay match a specific point on the chromaticity diagram, other conditionsmay be applied to select the optimal spectrum for the given filter inaccordance with the invention. For example, such a condition may bemaximum brightness for a given primary at a certain chromaticity point,which indicates that the primary may be an optimal stimulus of therelevant illumination, as explained, for example, in Gunter Wyszecki andW. S. Stiles, Color Science: Concepts and Methods, Quantitative Data andFormulae, 2d. Ed., 1982, pp. 179-183, which is incorporated herein byreference. The filter curve may have about 100% (or close to 100%)transmission in the pass band and about 0% (or close to 100%)transmission outside the pass band, and that the cut-off points may bedetermined so that the chromaticity of the resulting light is obtainedwith maximal brightness. Another condition may be that the filters arechosen is such a way that their positive linear combinations maysubstantially match the spectra of the inks and their overlaps. Thisensures that when colorimetric match is achieved, a spectral resemblancebetween the monitor and the print may also be achieved, reducingsensitivity to observer variability.

FIG. 3A schematically illustrates transmission spectra for a set offilters that may be used in accordance with some embodiments of thepresent invention. FIG. 3B illustrates positive (i.e., additive) linearcombinations of the three filters shown in FIG. 3A, which reproduce theperceived reflection spectra of the CMY inks and their overlaps.

The use of filters selected in accordance with embodiments of theinvention, for example, the transmission spectra illustrated in FIG. 3A,designed in such a way to obtain spectral resemblance, has anotherbenefit. By adding another set of filters that converts the lamp lightto standard illuminations (e.g. D50, D65, A, etc.), such that the samecolor temperature and a high color-rendering index is achieved, a goodrepresentation of the print under known lightning conditions isachieved. Furthermore, another set of filters that reproduces thereflection spectrum of typical substrates may also be inserted in thelight path, thus allowing for simulation of various print substrates. OnCRT monitors, this type of adaptations, e.g., to compensate forillumination conditions and substrate reflections, require profiling ofeach combination of substrate and illumination conditions. In theprojection device of the present invention, in contrast, such profilingis not required because the combination of the filters selected inaccordance with embodiments of the invention reproduces light thatsubstantially mimics the transmission spectrum of the inks and the lightreflected off the substrate. Thus, the filter selection in accordancewith embodiments of the invention simulates the process of viewing animage printed on a substrate, and thus new colorimetric adjustment,e.g., by profiling, is unnecessary. These features of substratesimulation and illumination adaptation without profiling are majoradvantages of a projection based proofing device in accordance withembodiments of the invention.

FIG. 7 schematically illustrates transmission spectra for a set offilters that may be used in accordance with further embodiments of thepresent invention. In the case that proofing in accordance with theinvention is performed in known illumination conditions, and for a giventype of paper, the transmission spectra of the three or more colorfilters may also account for the paper and illumination correctionfilter curves. In this embodiment, the primary color filters have a dualfunction of creating primary colors and correcting illumination, asdiscussed above. In FIG. 7, the spectra of three filters that may beused in such a projection display are shown. These filters are based onthe original filters designed for spectral match, e.g., the filters withtransmission spectra as illustrated in FIG. 3A appropriately modified toadjust the illumination of the high pressure mercury lamp usually usedin projector to that of a standard D50-like fluorescent lamp that may beused during proofing using the correction curve of FIG. 4C. This exampledemonstrates integration of the function of correction filters as partof the color filters. Such integration is advantageous, e.g., inreducing the number of optical surfaces, and thus reducing stray lightto improve contrast. The use of integrated filters also simplifies thedesign of the projection display and, therefore, reduces productioncosts.

Special color inks, e.g., Pantons, Toyo, etc., which cannot be proofedeven on prints produces by CMYK printers, and of course not on a CRTmonitor, may also be covered by selecting appropriate filters inaccordance with embodiments of the invention.

In some embodiments of the present invention, additional primary colors,for example, a yellow primary color, may be used to improve imagereproduction. The brightness of the yellow region of the spectrum thesensitivity of the human eye peaks at yellow. Furthermore, inhigh-pressure Mercury lamps, which are commonly used by projectiondisplays, most of the energy of the lamp is emitted within a few peaksub-spectra, one of which is the yellow region of the spectrum. In athree-primary system, yellow may be composed of green and red primarycolors and, therefore, very bright green and red colors are required inorder to achieve a sufficiently bright yellow. As illustrated in FIG.3B, the use of three primaries does may not allow for good spectralresemblance, particularly in the yellow region. Furthermore, the “dip”in the yellow spectrum shown in FIG. 3B may reduce the intensityproduced by the yellow peak of the lamp, thus reducing the deviceefficiency. Therefore, addition of yellow filters in accordance withsome embodiments of the invention may increase the efficiency of thesystem considerably. Additionally or alternatively, depending on thespecific filter selections, the addition of a yellow filter may furtherwiden the gamut of the device to include more special colors, whilemaintaining a reasonable efficiency. For example, if yellow is included,the green and the red spectra may be further shifted toward the edge ofthe chromaticity diagram thus allowing for an extended gamut that maycover more special colors.

Another possibility is to include two types of blue filters. In athree-primary display the brightness of the blue filter is much higherthan that of a typical blue elementary color of printed materials.Therefore, the blue light produced by a blue filter may be overly brightbecause its brightness may be used in reproducing white color. As aresult, when presenting the blue elementary color, or similar colors,only a small portion of the light through the blue filter is required,thereby potentially limiting the useful dynamic range of the device. Inorder to overcome the problem, in some embodiments of the invention, twotypes of blue filters may be used, one which is designed to providesufficient blue to reproduce the white, and another which has lowbrightness which allows to reproduce the full dynamic range of theprinted blue.

Another important advantage of projection displays in accordance withembodiments of the present invention is their relative insensitivity toambient illumination. The use of special viewing screens, for example,the Black Screen™, available from Denmark Visual Systems, CA, USA, orthe DNP Black Bead Screen™, available from DNP (Dai Nippon Printing Co.Ltd.), Denmark, allows for a very low reflection of ambientillumination, which may be, e.g., one order of magnitude lower than thereflection from a conventional CRT screen. At the same time, thebrightness of the projection display may be much higher than that ofCRT, depending on the light source, which determines the brightness. Thebrightness of images produced by embodiments of the present inventionmay be about 4 times higher than that of prior art soft-proofingdisplays, resulting in a ratio between the brightness of the colorcreated on the display and the reflected ambient light of 40:1 comparedto existing soft-proofers. The flexibility to increase the brightness bychoice of lamp further enables an increase of brightness and contrastthereby enabling comparison between a printed image and a displayedimage at a similar level of brightness. This advantage of embodiments ofthe invention obviates the need for perceptual adjustments.

Projection systems in accordance with embodiments of the invention allowfor stable, exchangeable, and durable color reproduction. The color ismostly controlled by the spectrum of the lamp and the color filterselections. The stability of interference filters, as described above,is very high. Furthermore, while in CRT monitors a balance must bemaintained between three different electron guns at all times, requiringa sophisticated electronic circuitry and continual profiling, thebalance between colors in the projection display of the invention isdetermined by the selected filters, assuming that the spectrum of thelamp is substantially stable throughout its useful life. Indeed duringthe useful life of the lamp, minor changes to its spectrum might occur,however, these changes are rather slow and therefore can be easilycalibrated, as described below. Furthermore, the light source may beeasily changed when exhausted, while the decay of phosphors is permanentand requires a change of the CRT itself.

Typically, the image data arrives in a CMYK format; however other dataformats may be used. A display according to embodiments of the presentinvention may use the same data. In a three-primaries display, thetransformation from CMYK to the levels/values required of the displaysprimaries may be performed colorimetrically in a straightforward manner.First, the color for the print primaries (the inks and the overlaps),e.g., at the corners of the hexagon in FIG. 5, under the relevantillumination, may be determined in terms of values in a tree-dimensionalcolor space, e.g., XYZ. Then, a printer model may be used to predict thecolor, in XYZ coordinates, for a given CMYK combination. The resultingcolor is then constructed in terms of the display primaries byconverting the XYZ values to corresponding values of the threeprimaries, e.g., using a 3×3 conversion matrix. ICC profiling, as isknown in the art, may also be performed in order to improve accuracy.Alternatively, conversion from the CMYK space to the three-primary spacemay be performed by matching the spectrum of the ink primaries and theiroverlaps. However, if suitable filters are chosen in accordance withembodiments of the present invention, the correct calorimetric match isexpected to also yield satisfactory spectral resemblance. In alternateembodiments, other suitable data formats and/or conversion techniquesmay be used.

The situation may be more complicated in the case of a display usingfour or more primaries. In the first steps the same process may beperformed, namely, a printer model may be used to predict the color ofthe CMYK combination. However, the color is determined by three values(e.g., XYZ), which must be converted to four or more parameters tocontrol the display colors. Several methods may be applied to solve thiscomplexity. One possible solution is to pre-set the values of thedisplay primaries that correspond to each of the inks and theiroverlaps. For each of the ink primaries and their overlaps (CMYRGB), aset of values is determined to define the display primaries required toreconstruct the ink primaries and their overlaps. Since there is morethan one combination per each primary, optimization may be performedaccording to additional constrains, e.g., maximal spectral resemblanceor minimum sensitivity to digitization noise. Since the values arepre-set, there is no problem running complicated algorithms to set them.When the data arrives, the CMYK values may be converted to correspondingink and overlap coverage, based on a printer model. Then the equivalentpre-set combination of the display primaries may be used to replace eachink primary (or overlap) in an amount determined by the ink coverage.Finally, an ICC profiling may also be applied to improve accuracy. Inalternate embodiments, other techniques may be used.

In practice, since the color filters, lamp and optical engine havecertain spectral tolerances, an initial calibration process may beperformed. This calibration enables maximum colorimetric and spectralmatch between the output of the monitor and any calibrated “master”monitor or printed material. As an output, the relative intensities ofthe primaries that contribute to each CMYRGB hexagon curve corner aredetermined.

During the operation period, slight changes in the spectral propertiesof the lamp may occur, although the changes of spectral properties ofthe optical elements within the operational time are typicallynegligible, and if such changes exist they can be compensated for byreal time calibration. A detector may be used to measure the luminousintensity of the lamp light as it passes through the three or moreprimary filters. If there is a change in the relative intensities of thelight source, colors, or primaries, e.g., the ratio of Blue divided byGreen and/or Red divided by Green in a three primary system, this changemay be measured and, for example, the result may be used to multiply (ordivide) the relevant intensities of each primary in the calculation thatdetermines the CMYRGB hexagon corners. It should be emphasized that, inembodiments of the present invention, there is no need to perform acalorimetric calibration, because the adjustment of the relativecolorimetric intensities would generally result in excellentcolorimetric accuracies for relatively large possible lamp spectralchanges. Also, there is no need for high absolute detector accuracy. Theonly value that should preferably be very accurate is the relativedetector response in the various spectral bands.

The advantages of the projection system described above are quite clear.First, it has a full coverage of the required gamut, and it may alsoinclude special colors that are outside the gamut of CMYK process inks.A change in the illumination conditions and the reflectance propertiesof the paper is easy to perform. The brightness level at which thecomparison between print and display is performed can be adjusted tolevels which are common in the industry for viewing prints. Finally, thedisplay in accordance with embodiments of the invention is relativelyinsensitive to ambient light, and thus can be used practically anywhere.The data flow is typically consistent with existing workflows in thesense that the device may accept CMYK data files. In this sense, thedevice of the invention may be used like a printer, however, itsimmediate response enables interactive proofing.

The benefits described above allow the projection display of theinvention to integrate smoothly into the pre-press and other publishingenvironments. The display may be used as a monitor for a computer, oralternatively it may be combined with a light box to create a fullsolution for comparing prints and proof-on-display under well definedambient light. This is due to the insensitivity of the display toambient light, which allows it to represent color accurately even understrong ambient illumination. The use of internal filters to adjust thelight allows the display to match accurately the properties of the printunder a light box illumination. It is emphasized that this type ofconfiguration, e.g., combined display and light box, may be useful for,e.g., press applications, where the operator of the device mayrepeatedly and efficiently match printed sheets and proofs.

Although the present invention is described herein in the context of asequential-projection type display device, it will be apparent topersons of ordinary skill in the art that the principles of theinvention, specifically, the selection of transmission spectra asdescribed herein, are also applicable and suitable for asimultaneous-projection type display device. For example, a projectiondisplay device in accordance with further embodiments of the inventionmay have three Spatial Light Modulators (SLMs) that operatesimultaneously to reproduce an image, each using modulating light of oneof the three primary color spectra defined above. In such devices, thelight from a polychromatic light source may be separated into thedesired spectra, using dichroic mirrors as are known in the art. Each ofthe colors may be passed through its respective SLM, which may include atransmissive liquid crystal SLM, and the modulated colored light maythen be combined and projected onto a viewing screen, as is known in theart. In some embodiments, additional color filters may be used in someor all of the primary light paths, to further control the primarycolors. The choice of dichroic minors and additional filters may be suchthat the gamut of the display will be as wide as or even wider Han thatof the print to be proofed, as explained in detail above.

While certain features of the invention have been illustrated anddescribed herein, many modifications, substitutions, changes, andequivalents will now occur to those of ordinary skill in the art. It is,therefore, to be understood that the appended claims are intended tocover all such modifications and changes as fall within the true spiritof the invention.

1. A display for reproducing an image intended for printing on asubstrate using a set of inks, the image having a perceived color gamutwhen printed on said substrate, the display comprising: a light sourcegenerating a set of at least three primary colors; and a controllercombining the set of at least three primary colors to substantiallyreproduce said image, wherein said at least three primary colors definea viewed color gamut which substantially covers said perceived colorgamut.
 2. The display of claim 1 comprising a correction filter, thespectrum of the correction filter being based on the spectrum reflectedfrom a type of said substrate.
 3. The display of claim 1 comprising acorrection filter, the spectrum of the correction filter being based onthe spectrum of an intended light used to view the image when printed.4. The display of claim 1 wherein the light source includes at least aplurality of LEDs.
 5. The display of claim 1, wherein the light sourceincludes at least a color wheel.
 6. The display of claim 1, wherein thelight source is able to produces at least four primary colors.
 7. Thedisplay of claim 1, wherein the light source produces three primarycolors, the transmission spectra of which define said viewed colorgamut.
 8. The display of claim 1 comprising a spatial light modulator.9. The display of claim 1 comprising a digital micro-mirror device. 10.A method for reproducing an image intended for printing on a substrateusing a set of inks, the image having a perceived color gamut whenprinted on said substrate, the method comprising: accepting datacorresponding to said image; converting said data into datacorresponding to a set of at least three primary colors; selectivelyproducing light of said at least three primary colors; and combining thelight of at least three primary colors to substantially reproduce saidimage, wherein said at least three primary colors define a viewed colorgamut which substantially covers said perceived color gamut.
 11. Themethod of claim 10 wherein converting said data comprises converting thedata using a conversion matrix.
 12. The method of claim 10 comprisingpassing light through a correction filter, the spectrum of thecorrection filter being based on the spectrum reflected from a type ofsaid substrate.
 13. The method of claim 10 comprising passing lightthrough a correction filter, the spectrum of the correction filter beingbased on the spectrum of an intended light source used to view saidimage when printed on said substrate.
 14. The method of claim 10comprising passing light through a color wheel.
 15. The method of claim10, wherein said at least three primary colors include a red primarycolor a green primary color and a blue primary color the transmissionspectra of which define said viewed color gamut.
 16. The method of claim10 comprising spatially modulating the light of said at least threeprimary colors.